When you installed Windows you ran Bootcamp correct and put in the drivers? If so there should be a item in Windows down by the clock and Volume control called Boot Camp. Click there and you should see an option which OS to boot from.

I'd really like to fix my mac os x but I dont know how and its really confusing.. can you help me fix it? The problem is i cant open finder and that means i cant excess any of my folder. my desktop seems blank. please help me

What version of OSX are you running? What model of mac are you running it on? Did it come with recovery discs? If you upgraded OSX from the version that came with your Mac, do you have the CD (if it came on a CD)? Are you making regular Time Machine backups?

If you have a CD, (if you upgraded to Snow Leopard or bought a computer originally running SL or earlier with a CD drive) and it the CD appears on your desktop when you insert it, you should be able to reinstall from a Time Machine Backup, provided you have one. If not, things get a bit more complex. You should be able to buy the recovery CDs for your model from Apple (I think) or a third party. Note that recovery CDs only work with your EXACT model of Mac (e.g. a CD for a Macbook 2,1 won't work with a Macbook 3,1). If you're running Lion or Mountain Lion, I believe (this is pure speculation) that you can redownload the installer from the Mac App Store.

There's a sizable chance the above paragraph is entirely incorrect as none of it is based on personal experience, but rather several things I've read online.

I'm using the macbook white with Mac OS X 10.5.8 I've lost the disc because I bought it a long time ago. But even if i have the disc, i cant do a thing because my desktop is clear and there's only the dock appears. The finder keeps restarting..